Comparative Ecology of Pinyon Mice and Deer Mice in Mesa Verde National Park Part 9

One _P. truei_ had a broken tail; three other individuals had tails about one-half normal length. One _P. maniculatus_ had a shortened tail.

Some of these injuries probably were caused by the Sherman live traps; several individuals of _P. truei_ were released after having been caught by the tail by the spring-loaded door of these traps.

On October 17, 1963, an adult _P. truei_ had a bleeding p.e.n.i.s; when this mouse was recaptured on October 25, the injury was healed.

Losses Attributed to Exposure in Traps

Observations of wild mice caught in live traps suggest that metabolic maturity is reached later than physical and reproductive maturity. In such trapping, it became apparent that juvenal and young mice suffered from exposure to cold and to heat much more than did subadult or adult mice. Although traps were carefully shaded and ample nesting material and food provided, some mice died in the traps. An overwhelming majority of these mice were juveniles and young.

Traps were checked in the morning, both in the summer and autumn, yet mice died in traps that were barely warm to the touch, in summer, and cool to the touch in autumn. Older mice frequently were found in traps that were warm, or even hot, to the touch; yet the older mice rarely died in such traps. Apparently the tolerance of adults is much greater to heating and chilling. Greater bulk and perhaps longer pelage in adults might provide sufficiently better insulation to account for this difference.

Occasionally juvenal mice were found in traps in a sluggish and weakened condition, especially in autumn when nights were cool. In such cases the mice were either cupped in the hands and warmed until lively enough to fend for themselves, or, if especially weakened, were taken to the laboratory. None of such animals that were returned to the laboratory lived for more than two weeks. Most of those released in the field did not reappear in the traps.

I conclude that juvenal and young mice placed under stress by overheating or cooling die immediately or live only a few days. Subadult and adult animals tolerate more extreme conditions of overheating or cooling, presumably because they are able to regulate their internal temperature better, by either losing or retaining heat more effectively.

Mice found dead in overheated traps had salivated heavily, and may also have licked the fur on their chests to increase heat dissipation. One such adult, of _P. truei_, had a wet chest when he was taken from a warm trap; when released, this mouse ran to a nearby plant of _Comandra umbellata_, and ate a few of the succulent leaves before running off.

This individual was trapped several times later in the summer, and apparently suffered no ill effects from the exposure.

Dental Anomalies

Abnormalities in the formation and occlusion, or decay of teeth, are relatively rare in wild mammals. Of all bodily structures, the teeth apparently are under the most rigid genetic controls; they form early in the embryo and follow rigidly specified patterns in their ontogeny.

Apparently any deviation from the normal pattern of tooth formation is quickly selected against. All specimens of _P. m. rufinus_ and _P. t.

truei_ in the collection of the Museum of Natural History at the University of Kansas, and in my collection, were examined for dental anomalies. A total of 317 specimens of _P. m. rufinus_ and 54 specimens of _P. t. truei_ were examined. The following specimens were found to have abnormalities:

K. U. 69361, _P. maniculatus_, adult: Small bundles of plant fibers are lodged between all upper teeth and have penetrated the maxilla anterior to the left M1. The maxillary bone is eroded away from the roots of all teeth. The anteriormost roots of both lower first molars are almost completely exposed, because the dentary has been abraded away.

K. U. 76041, _P. maniculatus_, young: A piece of plant fiber is wedged between the left M2 and M3. The maxillary bone has eroded away from around the roots of M3, indicating the presence of an abscess in this area.

K. U. 69362, _P. maniculatus_, adult: All teeth in the lower right tooth-row are greatly worn, especially on the lingual side. The l.a.b.i.al half of the right M1 is all that remains; decay is apparent both in the crown and roots on the lingual side of this tooth.

K. U. 69397, _P. maniculatus_, old: The maxillae have eroded away from around the anterior roots of each first upper molar, leaving these roots unsupported.

C. L. D. 231, _P. maniculatus_, old: The teeth in this female are greatly worn; re-entrant angles are not visible in any teeth. A circular hole, 0.1 millimeter in diameter, exists in the dentine immediately over (when viewed from the underside of the skull) the posterior root of the right M1. The crowns of the teeth are greatly reduced in height, and the dentine is thin.

Anomalies in the Skull

Wormian bones and other abnormalities in the roofing bones are noted, as follows:

K. U. 76090, _P. maniculatus_, young: The interparietal is divided; the divided suture is in line with the suture between the parietals. The interparietal is 7.8 millimeters long.

K. U. 76091, _P. maniculatus_, young: A wormian bone, 0.5 millimeter by 0.2 millimeter, lies between the anterior border of the interparietal and the posterior border of the left parietal, at a point midway between the center line of the skull and the posterolateral border of the parietal bone.

C. L. D. 248, _P. maniculatus_, adult: An oval wormian bone, 1.1 millimeters long and 0.6 millimeter wide, lies between the parietals at their posterior margin; the long axis of the bone is parallel to the long axis of the skull.

C. L. D. 246, _P. maniculatus_, juvenal: The interparietal is divided equally by a suture. An oval wormian bone, 0.3 millimeter long and 0.1 millimeter wide, lies between the frontals, midway between the anterior and posterior borders of these bones.

C. L. D. 656, _P. maniculatus_, young: A small, rounded wormian bone lies between the right parietal and interparietal, lateral to the posterior junction of the suture between the parietals. This bone extends anteriorly into the parietal bone from the suture of the interparietal and parietal. This bone is 0.7 millimeter wide, and extends 0.6 millimeter into the parietal.

C. L. D. 662, _P. maniculatus_, subadult: An elongated, diamond shaped wormian bone closes the suture between the parietal bones. This bone is 2.3 millimeters long and 0.8 millimeter wide.

K. U. 34735, _P. truei_, old: The anterior one-quarter of the left parietal bone is slightly depressed; and the posterior one-third of the left frontal and anterior one-quarter of the left parietal are thin and sculptured. This malformation of the roofing bones posterior to the orbit probably is not the result of a break, for the orbital part of the frontal bone is normal. The frontal-parietal sutures are in the normal positions on both sides of the skull.

The above-mentioned anomalies do not appear to be correlated with age or locality at which the specimens were taken. Apparently such anomalies are present throughout the population, but in a small percentage of specimens.

FOOD HABITS

Mice of the genus _Peromyscus_ are known to eat a wide variety of plants and arthropods, and to be highly opportunistic in selection of food (Cogshall, 1928; Hamilton, 1941; Williams, 1955, 1959a; Jameson, 1952; Johnson, 1962). In order to determine possible food preferences, captive mice of both species were fed plants indigenous to Mesa Verde. Entire plants were used whenever possible; available seeds also were offered (Tables 5, 6). All feeding experiments were replicated with at least six different individuals in order to minimize the trends resulting from individual preferences or dislikes. The mice of each species tended to be consistent in their feeding.

The plant species listed in Tables 5 and 6 were those that were eaten or rejected by a majority of the individuals tested.

Plant material eaten by _P. maniculatus_ and refused by _P. truei_ included only the leaves and stem of _Viguiera multiflora_. Plant material eaten by _P. truei_ and refused by _P. maniculatus_ included the leaves of _Calochortus gunnisonii_ and the leaves and stem of _Erigeron speciosus_.

TABLE 5--Plants, or Parts of Plants, Eaten by Captive Individuals of _P. truei_ in Mesa Verde National Park, Colorado. 0 = not eaten, + = eaten, - = not offered.

=============================+========+======+========+======= Species of Plant Leaves Stem Flower Seeds -----------------------------+--------+------+--------+------- _Amelanchier utahensis_ - - - + _Calochortus gunnisonii_ + + - + _Chaenactis douglasii_ 0 0 - - _Chrysothamnus depressus_ 0 0 0 - _Chrysothamnus nauseosus_ + 0 0 - _Comandra umbellata_ + + - - _Erigeron speciosus_ + + - - _Eriogonum alatum_ - - - + _Juniperus osteosperma_ - - - + _Lupinus caudatus_ 0 0 + - _Lithospermum ruderale_ 0 0 - 0 _Mellilotus alba_ + + + + _Mellilotus officinalis_ + + + - _Orthocarpus purpureo-albus_ + + + + _Pedicularis centranthera_ + + - - _Penstemon linarioides_ + + - + _Pinus edulis_ - - - + _Polygonum sawatchense_ + + - 0 _Solidago petradoria_ 0 0 0 - _Viguiera multiflora_ 0 0 0 0 -----------------------------+--------+------+--------+-------

Plant material eaten by captives of both species included _Calochortus gunnisonii_--stem and seeds; _Comandra umbellata_--leaves and stem; _Eriogonum alatum_--seeds; _Penstemon linarioides_--leaves and stem; _Pinus edulis_--seeds; and _Juniperus osteosperma_--seeds.

Plant materials refused by both species of mice included the leaves and stem of _Chaenactis douglasii_, the leaves, stem and seeds of _Lithospermum ruderale_, and the leaves, stem and flowers of _Solidago petradoria_.

Cricetine rodents chew plant and animal foods thoroughly; contents of their stomachs appear as finely-particulate fragments. These fragments invariably contain pieces of epidermis from ingested plants. Due to the presence of cutin in the cell walls, epidermis is last to be digested.

Microscopic a.n.a.lysis of plant epidermis is useful in helping to determine food habits of various animals (Dusi, 1949; Williams, 1955, 1959a; Brusven and Mulkern, 1960; Johnson, 1962). The microscopic a.n.a.lysis of stomach contents provides a practical method of determining which plants are eaten by rodents. Contents of stomachs and intestines were removed from mice caught in snap traps, and from preserved specimens. The contents were placed on a piece of bolting silk, washed thoroughly with running water, stained with iron-hematoxylin and mounted on slides, or stored in 70 per cent ethanol (Williams, 1959a; Douglas, 1965).

TABLE 6--Plants, or Parts of Plants, Eaten by Captive Individuals of _P. maniculatus_ in Mesa Verde National Park, Colorado. 0 = not eaten, + = eaten, - = not offered.

=============================+========+======+========+======= Species of Plant Leaves Stem Flower Seeds -----------------------------+--------+------+--------+------- _Artemisia ludoviciana_ 0 0 - - _Calochortus gunnisonii_ 0 + - + _Chaenactis douglasii_ 0 0 - - _Comandra umbellata_ + + - - _Erigeron speciosus_ 0 0 - - _Eriogonum alatum_ - - - + _Juniperus osteosperma_ - - - + _Lappula redowskii_ 0 0 - + _Lithospermum ruderale_ 0 0 - 0 _Orthocarpus purpureo-albus_ 0 0 + + _Penstemon linarioides_ + + + - _Pinus edulis_ - - - + _Purs.h.i.+a tridentata_ + + - - _Sitanion hystrix_ 0 0 - 0 _Solidago petradoria_ 0 0 0 - _Sphaeralcea coccinea_ + + - + _Stipa comata_ 0 0 - + _Viguiera multiflora_ + + - - -----------------------------+--------+------+--------+-------

In order to a.n.a.lyze these epidermal fragments, a collection of plants was made within the park. Slides of the epidermis of these plants were prepared and a.n.a.lyzed for diagnostic characters (Douglas, 1965:197-199).

Features such as the stomatal arrangement in relation to subsidiary cells; the types of trichomes, scales and glands; the cellular inclusions such as starch grains, mucilage and resins are of taxonomic value (Metcalfe and Chalk, 1950). The configuration of the anticlinal cell walls is useful in separating species that are similar in other respects (Douglas, 1965:199).

The following species of plants, and other food items, were identified in the stomach or intestinal contents of _Peromyscus maniculatus_:

_Agropyron smithii_ _Artemisia_ sp.

_Eriogonum umbellatum_ _Lupinus ammophilus_ _Penstemon linarioides_ _Phlox hoodii_ _Stipa comata_ Arachnid legs

Stomach and intestinal contents of _P. truei_ contained the following food items:

_Artemisia nova_ _Artemisia_ sp.

_Penstemon_ cf. _barbatus_ _Penstemon_ cf. _linarioides_ _Poa fendleriana_ Arachnid legs _Eriogonum_ sp.

_Gutierrezia sarothrae_ _Yucca_ sp.

Chitin Feathers